A large number of ranging devices have been proposed that project spot light onto an object for measurement, that receive the resultant reflected light, and that measure a distance to the object for measurement by trigonometrical ranging, as shown in FIG. 5. In the ranging device, as shown in FIG. 5, given that a center of a light emitting lens 2 is an origin O(0, 0), an axis of emitted light is y axis, and an axis orthogonal to the axis 5 of emitted light at the origin O is x axis, a light beam emitted from a light emitting element 1 placed on a point A(0, −d) is made into a generally parallel light beam by the light emitting lens 2 placed on the origin O, which beam projects a light spot on a point B(0, y) on an object 3 for measurement. The light beam reflected by the object 3 for measurement is condensed by a light receiving lens 4 placed on a point C(L, 0), is imaged on a point D(L+1, −d) on a position detecting element (e.g., PSD: Position Sensitive Detector) 6 placed on a line extending from the point A in a direction of the x axis, and thereby forms a received light spot.
On condition that a point at which a line passing through the point C, i.e., the center of the light receiving lens 4, and being parallel to the y axis intersects the position detecting element 6 is defined as point E (L, −d), a triangle OBC is homothetic to a triangle ECD. Accordingly, a distance y to the object for measurement 3 can be detected through detection of a position of the received light spot by the position detecting element 6, measurement of a side ED (=l), and calculation of expression (1) below.
                    y        =                              L            ·            d                    l                                    (        1        )            
This is a general principle of the trigonometrical ranging.
As the position detecting element 6, the PSD, a linear image sensor or an image sensor having a plurality of photodiodes placed thereon, or the like is used for detecting the position of an optical center of gravity of the received light spot projected onto the position detecting element 6.
Providing that the position detecting element 6 has a resolution of detection, Δl, a resolution Δy at the measured distance y can be expressed by equation (2).
                              Δ          ⁢                                          ⁢          y                =                                            -                                                y                  2                                                  L                  ·                  d                                                      ·            Δ                    ⁢                                          ⁢          l                                    (        2        )            
That is, selection of the position detecting element 6 that is to be used involves determination of the resolution Δl of detection of the position detecting element 6, and thus the resolution Δy at the measurement distance y is determined by a distance L (which will be referred to as a base line length, hereinbelow) between the light emitting lens 2 and the light receiving lens 4 and by a focal length d of the light receiving lens 4. The greater each of them is, the smaller the resolution Δy at the measurement distance y is and thus the higher resolution of distance detection is. On condition that a distance to a far object for measurement is measured, in particular, the measurement distance y is increased and thus it is necessary to increase the base line length L or the focal length d in order to reduce the resolution Δy.
FIG. 6 is a diagram showing a schematic configuration of a conventional optical ranging device. A light emitting element 12 and a light receiving element 13 are mounted in specified positions on a lead frame 11 and are individually sealed with a light permeable resin so that a light emitting side primary molded body 14 and a light receiving side primary molded body 15 are formed. Both the primary molded bodies 14 and 15 are sealed with a light shielding resin so that a secondary molded body 16 is formed. Then a lens holder 19 provided with a light emitting lens 17 and a light receiving lens 18 is fitted on the secondary molded body 16, and the optical ranging device is thereby formed.
Measurement of a distance to a far object for measurement by such an optical ranging device can be attained by increase in the focal length d of the light receiving lens 18, as shown in FIG. 7, or by increase in the base line length L that is a distance between centers of the light emitting lens 17 and the light receiving lens 18, as shown in FIG. 8. Both arrangements, however, cause increase in size of the optical ranging device as a whole and increase in costs thereof, necessitate a relevant space in electronic equipment to install the optical ranging device, and thus make the device difficult to handle or use.
In order to cope with such problems, ranging devices (distance measuring devices) as disclosed in JP H07-98205 A (Patent Literature 1) and JP 2011-145115 A (Patent Literature 2) have been proposed.
In the ranging device of the above-mentioned Patent Literature 1, as shown in FIG. 9, an optical path changing means composed of two pairs of mirrors 24B, 25B, 24R, 25R for changing two optical paths of beams of incident light from an object for distance measurement so as to direct the beams toward a center axis of the device and so as to make the beams pass through a first lens 22B and a second lens 22R, respectively, is provided outside a ranging module 21 including the two light receiving lenses 22B, 22R and two light sensors 23B, 23R. Increase in the base line length B in that way allows precise measurement of distance values.
As shown in FIG. 10, the ranging device of the above-mentioned Patent Literature 2 includes a lens array member 26 having a pair of ranging lenses 26a, 26b, a mirror array member 28 having a pair of reflection members 28a, 28b, and an intermediate mirror member 29. The mirror array member 28 and the intermediate mirror member 29 are provided to reflect image forming beams coming through the ranging lenses 26a, 26b so that an image of an object is formed on each of imaging regions 27a, 27b of an imaging element 27. This arrangement allows measurement of precise distance values by an increase in focal lengths of the lenses and cancellation of a change in the base line length that is caused by change in temperature.
These conventional ranging devices, however, have problems as follows.
In the ranging device of Patent Literature 1, the optical path changing means is provided outside the ranging module 21 including the two light receiving lenses 22B, 22R and the two light sensors 23B, 23R. Accordingly, light, of which optical paths have been changed by the optical path changing means, is condensed by the light receiving lenses 22B, 22R. This raises a problem in that size of the ranging device including overall optical systems is consequently increased.
In the ranging device of Patent Literature 2, an optical system has a combination of the lens array member 26 and the mirror array member 28, and only one lens surface of each lens has a condensing effect (curved surface). Therefore, a sufficient condensing effect cannot be expected relative to the lens diameter (lens size) of the ranging lenses 26a, 26b and there is a fear that ranging precision may be decreased by insufficient quantity of light. In addition, a problem is caused in that the large focal length of the optical system will increase the size of the ranging device.
Besides, the provision of the mirror array member 28 and the intermediate mirror member 29 as reflectors involves difficulty in adjustment of positions thereof. There are different media, i.e., plastics material and air between reflection surfaces 28c, 28d of the reflection members 28a, 28b of the mirror array member 28 and reflection surfaces 29a, 29b of the intermediate mirror member 29. Thus a problem is caused in that wrong adjustment of the positions may cause optical faults, such as attenuation, refraction and surface reflection, on interfaces between the plastics material and the air.